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Unit 3.2: The Periodic Table and Periodic Trends Notes
The Organization of the Periodic Table
Dmitri Mendeleev was the first to organize the elements by their periodic properties. In 1871 he arranged the elements in vertical columns by their atomic mass and found he could get horizontal groups of 3 or 4 that had similar properties. Mendeleev discovered a repeating pattern or periodic trend in the elements that were known at the time. He was able to predict properties of elements that were not yet discovered. In some cases Mendeleev’s table had some irregularities. Putting elements in order of increasing atomic mass put elements in column where they didn’t seem to fit (Te and I). Mendeleev thought the masses must be wrong, but he was wrong!
[Mendeleev’s Periodic Table] [Electron Configuration Periodic Table]
Henry Moseley later discovered that each element has a unique nuclear charge. The nuclear charge is the total charge of all the protons in the nucleus, which has the same value as the atomic number. When he arranged in order of atomic number instead of atomic mass the irregularities Mendeleev found disappeared.
THE PERIODIC LAW : Physical and chemical properties of the elements are periodic properties of their atomic number.
The elements are arranged in the table by their electron configurations. Elements in the same vertical column are called families or groups. Groups have similar properties and the same arrangements of valence electrons in their outer electron shell. Elements in the same horizontal row are called periods. Periods have the same major energy level.
The periodic table provides a map for all the elements: Metals – solids except for Hg mercury; good conductors, shiny, malleable Nonmetals – gases or brittle solids Metalloids – along the “stairstep” Noble gases – nonreactive gases, monoatomic, almost inert group VIIIA (or sometimes 18)
The Groups of the Periodic Table
ALKALI METALS
Group 1 is the alkali metals. These are soft metals whose outer electron shell has an s 1 configuration. These are the most active metals. They tend to react quickly with air or water, producing a basic solution in water. They lose the s 1 electron and become ions with a +1 positive charge. Notice that if this happens they have the electron configuration of a noble gas.
ALKALINE EARTH METALS
Group 2 is the alkaline earth metals. Their outer electron shell has an s 2 configuration. They are harder & less reactive than the Group 1 metals. They lose the s 2 electron and become ions with a +2 positive charge. Notice that if this happens they have the electron configuration of a noble gas.
Periodic Trends
Horizontal & vertical trends can be seen in the elements for:
- atomic radius
- ionization energy
- electron affinity
- electronegativity
ATOMIC RADIUS
To find atomic radius, atoms are assumed to be spheres. The electron cloud size determines the atomic radius for an atom. The radius values are only estimates. These values are measured by finding the distance between 2 nuclei and dividing the distance by 2.
GROUP TREND : Atomic radius increases as you move from top to bottom in a family. This is because major energy levels (1-7) are being filled with more & more electrons. The electrons get farther & farther from the nucleus.
PERIOD TREND : Atomic radius generally decreases from left to right as atomic number increases. This is because extra electrons are entering the same level while the nucleus gets larger & more positive. This draws the electron cloud in towards the nucleus.
[Atomic Size of Elements] [Atomic Size of Ions]
ATOMIC RADIUS OF IONS: When an atom loses an electron it has a positive charge. The radius of the atom decreases because there’s a smaller electron cloud. When an atom gains an electron it has a negative charge. The radius of the atom increases because the electron cloud is larger.
IONIZATION ENERGY
Ionization energy is the energy needed to remove an electron from an atom.
GROUP TREND : In vertical groups, ionization energy decreases from top to bottom. This is because electrons are farther from the nucleus & filled levels cause a shielding effect.
SHIELDING EFFECT: Inner electrons shield outer electrons from the positive nucleus. This means outer electrons are not held as tightly.
PERIOD TREND : Ionization energy tends to increase as you move from left to right toward the noble gases. This is because metals tend to lose electrons & nonmetals tend to gain electrons. All of them want to as stable as the noble gases.
[Diagram of Ionization Energy] [Diagram of Electron Affinity]
ELECTRON AFFINITY
Electron affinity is the ability of an atom to attract and hold an extra electron.
GROUP TREND : Electron affinity decreases from top to bottom of a group. This is because it’s easier for small atoms with the nucleus closer to the outer electrons to gain another electron.
PERIOD TREND : Electron affinity in a horizontal period increases from left to right. This is because the desire to gain an electron increases the closer you get to fill the energy level. What do you think the electron affinity of the noble gases is? Zero, they are happy like they are.
